The present invention relates to dispersions for preparing roof tile paints, to roof tile paints, and to roof tiles coated with roof tile paints.
In the course of the production of concrete roof tiles, a mortar mass is shaped and then generally colored, usually prior to curing, by coating with an emulsion paint (i.e., in the form of a dispersion). Subsequent curing then takes place together with the drying of the paint at temperatures of from 40 to 100xc2x0 C. After curing, there may be a second coating with an emulsion paint or with a clearcoat, and subsequent drying.
In order that the paint does not coagulate on the uncured concrete, which is referred to as green concrete, the binder is required to possess a certain degree of stability, e.g. cement compatibility and heightened salt stability. This is generally achieved through the use of ionic and/or nonionic emulsifiers, and also by using functional monomers, such as methacrylic acid, acrylic acid, maleic acid, acrylamide, methacrylamide, ethenesulfonate and/or sulfoxyethyl methacrylate, for example. In addition to coloring, the emulsion paint has the function of preventing lime efflorescence as the concrete roof tiles cure. Another function is to prevent or reduce soil pickup, and in particular the growth of algae, on the finished roof tiles.
The use of dispersions based on (meth)acrylic esters and/or styrene for preparing roof tile paints is already known from DD 124808. EP-A-0 492 210 describes the use of certain monomers for reducing the soil pickup tendency of the concrete roof tile. For the purpose of reducing lime efflorescence, EP-A-0 469 295 claims the use of emulsifiers based on sulfonated diaryl ethers.
For the purpose of stabilization, the dispersions include hydrophilic constituents, such as emulsifiers and functional monomers such as methacrylic acid, acrylic acid, maleic acid, acrylamide, methacrylamide, ethenesulfonate or sulfoxyethyl methacrylate, for example. Owing to the hydrophobic constituents, the corresponding paint films exhibit heightened water absorption, which may lead to increased algal growth on the concrete roof tiles and, under conditions of freeze/thaw cycling, to cracking and flaking of the paint owing to the water absorbed.
For the purpose of preventing algal growth, DE-A-39 01 073 describes the use of copolymerizable tin compounds. Like the subsequent addition of algicides, however, this is ecologically objectionable.
It has now surprisingly been found that, by using carboxymethylcellulose as protective colloid, it is possible to prepare dispersions which, as binders in roof tile paints, exhibit sufficient cement stability but low water absorption in the paint film, leading to reduced algal growth, improved freeze/thaw stability of the coating, and enhanced protection against efflorescence.
The present invention provides for the use, for preparing roof tile paints, of a dispersion prepared using from 0.1 to 3, preferably from 0.1 to 1.5, percent by weight (based on the total monomer content) of carboxymethylcellulose as protective colloid, in connection with which a monomer composition comprising
a) from 85 to 99.8% by weight of acrylic esters with C1 to C12 alkanols, methacrylic esters with C1 to C12 alkanols and/or vinylaromatic monomers,
b) from 0.2 to 5% by weight of copolymerizable carboxylic acids, carboxamides, sulfates and/or sulfonates, and
c) from 0 to 10% by weight of other monomers
is polymerized in the form of an aqueous emulsion.
The carboxymethylcellulose used preferably comprises water-soluble carboxymethylcelluloses having a degree of substitution of from 0.4 to 2.9, with particular preference from 0.4 to 1.5, and in particular from 0.6 to 1.4, the degree of substitution referring to the average number of carboxymethyl groups introduced per anhydroglucose unit. It is preferred to use carboxymethylcelluloses whose 2% strength aqueous solutions possess a Brookfield viscosity (20 rpm) at 25xc2x0 C. of less than 2000 (with spindle 3), with particular preference less than 500 (with spindle 2), and in particular less than 100 (with spindle 1) mPas. The carboxymethylcellulose is used preferably in the form of its ammonium salt or alkali metal salt. Examples of suitable commercial products are Blanose 7M(copyright), Blanose 7UL(copyright), Blanose 7EL(copyright) and Ambergum 3021(copyright) from Aqualon. The carboxymethylcelluloses of the invention may also comprise further constituents, especially alkyl or hydroxyalkyl radicals having 1 to 4 carbon atoms, alkyloxyalkyl radicals having 2 to 6 carbon atoms, or dialkylamino radicals having a total of 2 to 6 carbon atoms. Examples of suitable compounds are methylcarboxymethylcellulose, ethylcarboxymethylcellulose, hydroxyethylcarboxymethylcellulose, hydroxypropylcarboxymethylcellulose, methoxyethylcarboxymethylcellulose, ethoxyethylcarboxymethylcellulose and diethylaminocarboxymethylcellulose.
Monomers a) used comprise acrylic and methacrylic esters of C1 to C12 monoalcohols, such as ethyl acrylate, butyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, butyl methacrylate, cyclohexyl methacrylate and isobornyl methacrylate, and/or vinylaromatic monomers such as styrene or vinyltoluene, for example. Use is made of those combinations, known to the skilled worker, of softening monomers, such as butyl acrylate and 2-ethylhexyl acrylate, for example, with hardening monomers, such as methyl methacrylate and styrene, for example, so that the glass transition temperature of the corresponding copolymer is situated preferably in the range from xe2x88x9210 to 60xc2x0 C., with particular preference in the range from xe2x88x925 to 50xc2x0 C., and in particular in the range from 0 to 40xc2x0 C.
If operating by the technique of multistage emulsion polymerization, the hardening and softening monomers and the ratio of the phases should preferably be combined such that the dispersion possesses a minimum film-forming temperature in the range from 0 to 50xc2x0 C. and the corresponding dispersion films possess an elongation at break of more than 150% for a film thickness of 100 xcexcm.
As far as copolymerizable carboxylic acids, carboxamides, sulfates and/or sulfonates b) are concerned, preference is given to the use of acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide, sulfoalkyl (meth)acrylates, such as the potassium salt of sulfopropyl methacrylate (=SPM from Roschig) and/or sulfoalkyl(meth)acrylamides, such as the sodium salt of acrylamido-2-methylpropanesulfonic acid (=AMPS from Lubrizol), for example.
Monomers which may be used as said other monomers c) are monomers containing keto groups, such as monomers containing acetoacetoxy groups, for example, examples being acetoacetoxyethyl methacrylate, acetoacetoxybutyl methacrylate, acrylamidomethylacetylacetone and vinyl acetoacetate, and polymerizable derivatives of diacetone, such as diacetoneacrylamide and diacetonemethacrylamide. Further compounds which may be used as monomers c) are hydroxyalkyl(meth)acrylates, glycidyl(meth)acrylates, alkoxyvinylsilanes, (meth)acryloyloxyalkylsilanes, (meth)acryloyloxyalkyl phosphates, and polymerizable ethyleneurea derivatives, such as N-(xcex2-(meth)acryloxyethyl)-N,Nxe2x80x2-ethyleneurea and N-(xcex2-acrylamidoethyl)-N,Nxe2x80x2-ethyleneurea, for example.
In order to improve the soil pickup behavior, dispersions whose other monomers c) include monomers containing keto groups may be admixed with polyfunctional carbohydrazides containing at least two hydrazide groups, such as adipohydrazide, oxalohydrazide, isophthalohydrazide and polyacrylopolyhydrazide, for example. It is preferred to use an equimolar ratio of hydrazide groups to keto groups.
The polymerization is conducted in accordance with the common techniques of emulsion polymerization, in which the monomers are emulsified in the aqueous phase in the presence of emulsifiers, initiators and the protective colloid and are polymerized at temperatures from 60 to 95xc2x0 C. The emulsion polymerization may be conducted in accordance with the common techniques, known to the skilled worker, such as batch, monomer metering or emulsion feed techniques. It is preferred to operate in accordance with the emulsion feed technique, in which a small amount of the monomers is prepolymerized and then the remaining amount of monomers is metered in as an aqueous emulsion. If desired, two or more different monomer emulsions may also be metered in successively. Like the emulsifier, the protective colloid may be included in part in the initial charge to the reactor and/or metered in together with the monomer emulsion.
An essential feature is that the polymerization is conducted at pH  greater than 3, preferably  greater than 5, with particular preference in the pH range from 5 to 8. For this purpose, the monomer emulsion, comprising the protective colloid and also the copolymerizable carboxylic acids, carboxamides, sulfates and/or sulfonates b), and/or the initial reactor charge, are/is adjusted using an aqueous ammonia, alkali metal and/or alkaline earth metal hydroxide solution to a pH  greater than 3, preferably  greater than 5, with particular preference to a pH in the range from 5 to 8. An alternative option is to use an appropriate buffer.
The preparation of high-quality dispersions in accordance with the present invention presupposes the application of the existing experiences in the field of emulsion polymerization, even where they are not described here. Failure to (properly) observe the rules known to the skilled emulsion polymerization worker may therefore adversely affect important properties, an example being the water resistance of the dispersion films. Based on the polymer content, therefore, the dispersions should not substantially exceed the commonly used amounts of up to 3% by weight, preferably up to 2% by weight, of ionic emulsifiers, and, respectively, up to 6% by weight, preferably up to 4% by weight, of nonionic emulsifiers.
Particular preference is given to the use of ionic emulsifiers only, in an amount of  less than 2% by weight.
Examples of nonionic emulsifiers which may be used are alkyl polyglycol ethers such as ethoxylation products of lauryl, oleyl or stearyl alcohol or of mixtures such as coconut fatty alcohol; alkylphenol polyglycol ethers such as ethoxylation products of octylphenol or nonylphenol, diisopropylphenol, triisopropylphenol or of di- or tri-tert-butylphenol; or ethoxylation products of polypropylene oxide.
Suitable ionic emulsifiers are primarily anionic emulsifiers. These may comprise the alkali metal salts or ammonium salts of alkyl-, aryl- or alkylaryl-sulfonates or -phosphonates, alkyl, aryl or alkylaryl sulfates or phosphates, or compounds with other anionic end groups, the presence of oligoethylene oxide or polyethylene oxide units between the hydrocarbon radical and the anionic group also being possible. Typical examples are sodium lauryl sulfate, sodium undecyl glycol ether sulfate, sodium lauryl diglycol sulfate, sodium tetradecyl triglycol sulfate, sodium octylphenol glycol ether sulfate, sodium dodecylbenzenesulfonate, ammonium tri-tert-butylphenol pentaglycol or octaglycol sulfate.
To start and continue the polymerization, use is made of oil-soluble and/or preferably water-soluble free-radical initiators or redox systems. Suitable compounds include, for example, hydrogen peroxide, potassium, sodium or ammonium peroxodisulfate, dibenzoyl peroxide, lauryl peroxide, tri-tert-butyl peroxide, bisazodiisobutyronitrile, alone or together with reducing components, examples being sodium bisulfite, Rongalit, glucose, ascorbic acid, and other compounds possessing reductive activity. Preference is given to the use of peroxodisulfates.
It is also possible to use regulators, such as mercaptans, especially n-dodecylmercaptan, thiophenol and/or 2-methyl-5-tert-butylthiophenol. Usually, amounts of from 0 to 1 % by weight, preferably from 0 to 0.5% by weight, are used, and with particular preference the dispersion is prepared without using a regulator.
The dispersions are usually adjusted to a pH of from 6.5 to 10, preferably from 7.0 to 9.0, using aqueous ammonia, alkali metal and alkaline earth metal hydroxide solutions.
If desired, the dispersion may further comprise film-forming auxiliaries, such as white spirit, Texanol(copyright), butyl diglycol and butyl dipropylene glycol, plasticizers, such as dimethyl phthalate and dibutyl phthalate, dispersants such as polyacrylic acids and corresponding copolymers, such as Lopon 890(copyright), Dispex G40(copyright), thickeners based on polyacrylates or polyurethanes such as Borchigel L75(copyright) and Tafigel PUR 40(copyright), preservatives, defoamers, such as mineral oil or silicone defoamers, for example, wetting agents, such as aminomethylpropanol, and other additives customary in the formulation of coating compositions.
The dispersion is suitable for coating roof files, preferably green roof tiles. Suitable coating compositions are prepared in a known manner by incorporating inorganic fillers and color pigments into the dispersion, it being possible to establish the desired viscosity of the paint by adding water and/or by using thickeners. Examples of suitable inorganic fillers include calcite, chalk, quartz flour and/or heavy spar. Suitable pigments include inorganic pigments such as red iron oxide, for example, and black pigments, and also organic pigments.